1. Field of the Invention
The present invention is generally related to an interlock switch and, more specifically, to an apparatus for sensing the closure of a door as indicated by the insertion of a key structure into a housing structure.
2. Description of the Prior Art
In many types of machine tools, it is advantageous to provide a means for assuring that a protective enclosure is closed prior to permitting the operation of the equipment. In some instances, this closure sensing operation is necessary to protect the safety of the machine operator by preventing the operator from being endangered by moving components. In other applications of devices of this type, the enclosure sensing equipment is utilized to protect damage from occurring to one portion of the equipment by other portions of the equipment that could occur if the machine was operated without all of the intended protective components being in their proper place.
Several types of enclosure sensing devices are known to those skilled in the art. Typically, a moveable component is moved toward a stationary component and some means is provided to determine that the moveable component has properly been disposed at a predetermined position relative to the stationary component. In some instances, the proper position of the moveable component relative to the stationary component is merely detected by a signal provided by the sensing apparatus. In other cases, the provision of the output signal is accompanied by some type of mechanical means that rigidly latches the moveable component to the stationary component.
U.S. Pat. No. 4,580,062, which issued to MacLaughlin on Apr. 1, 1986, discloses a safety shield control device that provides an electrical interlock to prevent unauthorized access to potentially hazardous equipment. An associated detection circuit is provided to detect if a safety shield or closure member is opened, not securely closed or has been tampered with. The detection circuit comprises a balanced bridge incorporating the electrical interlock into a leg thereof and may also provide a visual or audible indication whether the bridge is unbalanced.
U.S. Pat. No. 4,210,889, which issued to Holce on Jul. 1, 1980, describes a magnetically actuating sensing device that is intended for use in security monitoring systems. A switch unit is provided with a plurality of electrically interconnected magnetic Reed switch that are disposed in a predetermined physical relationship in order to provide for controlling electrical circuits. Each Reed switch is biased in a magnetically actuated state by a permanent biasing magnet in close proximity thereto. The biasing magnets are arranged in a predetermined combination of plurality orientations. A corresponding number of permanent actuating magnets with plurality orientators opposing those of the biasing magnets are disposed within an actuating unit so that they overcome the effect of the biasing magnets when the switch unit and actuating unit are in a predetermined physical relationship with each other.
U.S. Pat. No. 4,349,814, which issued to Akehurst on Sep. 14, 1982, describes an electrical switch for use on machinery to prevent machine operation and less safety guards or the like are correctly positioned. It comprises two separate parts. One part includes at least two Hall effect devices which respond to magnetic poles of opposite polarity and electronic switching means which is actuated only when all of the Hall effect devices are triggered simultaneously. The other part of the device includes a corresponding number of magnets that are positioned to actuate the respective Hall effect devices when the two switch parts are placed in close proximity and predetermined orientation.
U.S. Pat. 4,812,674, which issued to Sue et al on Mar. 14, 1989, discloses a safety gate limit switch that uses Hall effect transducers. The proximity switch has at least three Hall effect magnetic field sensitive transducers that are mounted in a transducer housing and has at least two magnets mounted in a magnet housing. The transducers produce logic signals and a logic circuit responsive to those signals activates an electrical switch only when the magnets are in a proper position relative to the transducers. The electrical switch may be used to control a machine and the machine is not activated until the proximity switch is activated.
U.S. Pat. No. 4,945,340, which issued to Brill on Jul. 31, 1990, describes a taper resistant magnetic security system. The system is intended for use in a physical security monitoring environment and it includes a switch unit that has a common conductor, a guard conductor and at least three switches. Each of the switches has a deactivated condition and an activated condition and each is adapted to be placed in its activating condition in response to a magnetic field of predetermined magnetic flux. The switch unit also includes a logic circuit electrically interconnecting the switches and the common and guard conductors, the logic circuit completing a series circuit between the common conductor and the guard conductor whenever at least two predetermined but all switches are in magnetically actuated condition.
U.S. Pat. No. 4,199,741, which issued to Serrus Paulet on Apr. 22, 1980, discloses a moving magnet rotary switch that comprises a stationary body that is provided with longitudinal bores in each of which a core of magnetic material is moveable. It also comprises switching means located at one end of the bores which are adapted to be actuated by the cores and magnetic elements adapted to be brought successively opposite the ends of said bores to either displace or hold the cores in the bores for actuating the switching means.